The hydrocarbons recovered from the Fischer-Tropsch synthesis reactor usually may be classified into three categories based upon a combination of their molecular weight and boiling point. The lowest molecular weight fraction is normally gaseous at ambient temperature and also the least valuable commercially. Parts of this fraction, which are usually collected as overhead gases, may be sold as LPG, and/or upgraded by oligomerization to higher molecular weight material, or recycled to the Fischer-Tropsch synthesis unit. The Fischer-Tropsch condensate fraction which usually has a boiling range between about ambient temperature and about 750 degrees F. is normally liquid at ambient temperature and may be processed into liquid fuels intended for the transportation fuel market, such as, naphtha, jet, and diesel or used in petrochemical processing, such as ethylene cracking. The Fischer-Tropsch wax fraction which is generally a solid at ambient temperature may be cracked to yield lower molecular weight material suitable for use as liquid fuels or may be processed to yield lubricating base oils.
While the lubricating base oils derived from the Fischer-Tropsch wax fraction have a high potential commercial value, the processing schemes required to make their conversion to lubricating base oil generally require a high initial capital investment and involve high operating expenses. Therefore, most commercial processing schemes for Fischer-Tropsch wax crack the wax to yield lower value liquid fuels in order to avoid the high costs involved. The present invention is directed to an integrated process for producing both liquid fuels and high quality lubricating base oils. The integrated process of the invention lowers both the initial capital costs of the processing equipment and the high operating costs of the unit by processing the Fischer-Tropsch condensate fraction and the wax fraction in separate but fully integrated processing trains. The present processing scheme also makes it possible to operate each of the various processing steps under optimal process conditions which increase the yields of the highest value products.
The separate processing of heavy and light Fischer-Tropsch fractions has been proposed for the production of liquid fuels in U.S. Pat. Nos. 5,378,348 and 6,589,415. The separate processing of lubricating base oils and liquid fuels is proposed in U.S. Pat. No. 6,432,297. The optimization of the process conditions in each of the hydrocracking unit, dewaxing unit, and hydrofinishing unit during the production of lubricating base oil is taught in U.S. Pat. No. 6,337,010. However, none of the earlier processing schemes are able to take full advantage of the synergies associated with employing optimal processing conditions in the separate processing trains for the Fischer-Tropsch wax fraction and the Fischer-Tropsch condensate fraction.
As used in this disclosure the words “comprises” or “comprising” are intended as an open-ended transition meaning the inclusion of the named elements, but not necessarily excluding other unnamed elements. The phrases “consists essentially of” or “consisting essentially of” are intended to mean the exclusion of other elements of any essential significance to the composition. The phrases “consisting of” or “consists of” are intended as a transition meaning the exclusion of all but the recited elements with the exception of only minor traces of impurities.